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For nearly two decades, scientists have noted dramatic changes in arctic tundra habitat. Ankle-high grasses and sedges have given way to a sea of woody shrubs growing to waist- or neck-deep heights. This shrubification of the tundra challenges animals like caribou that are adapted to low-stature arctic vegetation. Pinpointing a cause has been difficult for researchers.

Now, research from the University of Minnesota has found that regardless of soil fertility or rainfall amounts, the single variable that was by far the strongest determinant of how much a shrub grew in a given year was the temperature in June. The warmer the June temperatures, the faster the shrubs grow.

According to Daniel Ackerman, a Ph.D. candidate in the Department of Ecology, Evolution and Behavior in the College of Biological Sciences at UMN who led the research team, other variables, including temperatures during the rest of the growing season in July and August, barely had an impact on shrub growth.

Ackerman's team collected hundreds of shrub stems, which, like trees at lower lattitudes, form a concentric ring around their stem each growing season, and used these rings to measure the size of each annual ring in their samples. After measuring the rings using a microscope, the team created records of historic shrub growth from across northern Alaska. Next, the team compared these growth records to climate observations, examining factors like precipitation, temperature and solar radiation.

"Our new understanding of the link between June temperature and shrub growth means that we can expect shrubification to continue throughout northern Alaska," said Ackerman. "With this study and others like it, we're beginning to understand the causes of shrubification. However, we still have a ways to go in predicting its effects. It seems like larger shrubs will benefit some animals, like moose and ptarmigan, while other animals, like caribou, could be harmed."

Ackerman's research was funded in part by a National Science Foundation Graduate Research Fellowship. (Date image taken: 2016; date originally posted to NSF Multimedia Gallery: June 11, 2018)

Credit: Courtesy Daniel Ackerman

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